Earlier quoted context omitted.
Gory details are at https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/14478... It actually mounts root correctly, but them fails to mount home which has exactly the same details except a different subvol parameter.
Systemd seems to have ongoing problems with mounting...
In this case it would perfectly if it just tried to do the mount, exactly as written in fstab. Instead it tries to be clever and ends up outwitting itself.